Commercial optical fiber connector assemblies 100, such as shown in exploded view in FIG. 1, are used to couple optical fibers together so that light transiting from a bundle 102 of one or more fibers in one end 104 of the connector assembly 100 will pass through the connector assembly 100 to fibers or a device connected to the other end 106 of the connector assembly 100. A ferrule 108 is typically part of the connector 100 and is the part of the connector 100 into which the fibers 110 themselves are inserted before the ferrule 108 is inserted into the overall connector assembly 100 itself. The ferrule 108 holds the fiber(s) 110 in a precise position and ensures that when the connector assembly is attached to a mating connector assembly or some other device, the fibers of the connector assembly are held in consistent alignment.
In the multi-fiber connectors available today, such as shown in U.S. Pat. No. 5,214,730, most of the connections are for fiber arrays of between 2 and 12 fibers arranged in a single row (although some commercial 2×12 configurations are available). Those connectors are referred to by various names, depending upon who makes them. In 1×2 arrays, connectors are referred to as ST, LC, MT-RJ connectors while the 1×12 and some 2×12 array connectors are referred to as MTP®, MPO, MPX and SMC connectors, among others. In the 1×12 or 2×12 area, all of the various connectors use a common type of ferrule commercially available from, among others, US Conec Ltd. and Alcoa Fujikura Ltd. Moreover, in some cases, the ferrules used in the small array connectors (i.e. for less than 12 fibers) are form and fit compatible for use with the MTP, MPO, MPX and SMC connectors. In addition, other types of commercial connectors for small arrays of fibers (i.e. less than 12) are available or have been proposed, for example, as shown in U.S. Pat. No. 5,743,785.
FIG. 2 is an enlarged photograph, in perspective view, of a prior art 1×12 ferrule 200 having an outer dimensional shape for use in an MTP, MPO, MPX or SMC connector of the prior art. Such ferrules 200 are made by molding plastic or epoxy. For example, the 1×12 (shown) and similar 2×12 ferrule technology currently in commercial use is based upon molding and curing of a glass filled epoxy resin (a high-performance plastic) using a common molding technique called transfer molding.
There has been an increasing need among users in the fiber optic field for larger groups of fibers, so demand for single connectors to handle arrays of fibers in excess of 12 has been increasing as well. Today, ferrules 200 such as shown in FIG. 2 that are molded out of epoxies or plastics can be made to the necessary tolerances for small arrays of multimode fibers, on the order of one or two rows of up to 12 fibers each, but special care must be taken during fabrication. Plastic molding technology is very process sensitive and molds having the requisite precision for even small arrays are extremely difficult to make. Even so, yields tend to be poor due to the inherent manufacturing process errors that occur in plastics molding. Since the tolerances on these pieces must be very accurate (on the order of about 1 to 2 microns), high yield manufacture is difficult when the array size necessitates two rows and exceptionally difficult for more than two rows.
The overall ferrule volume is very small, since ferrules 200 for the above MTP, MPO, MPX or SMC connectors are about 2 mm (2000 microns) high, 6 mm wide and 8 mm deep, and have a face portion of at least 3 mm thick to support and hold optical fibers, so molding or machining of features into the face surface 202 of the ferrules through the face portion, in the number and size required to hold multiple optical fibers (which typically have about a 125 micron cladding diameter for both multimode fiber and single mode fiber and are spaced from each other on a center-to-center spacing (“pitch”) of 250 microns), is very difficult.
Additionally, making ferrules for larger arrays is made even more difficult because, as the holes approach the periphery of the ferrule, the structural integrity of the peripheral walls near the holes decreases. In addition, process variations during production cause parts to also have poor tolerance at the periphery. As a result, they become overly fragile, causing hole and in some cases component collapse and/or they have distortions or excess material that impedes or prevents fiber insertion and are too fragile to successfully attempt removal of any such material. The problem is that in molding plastic ferrules for holding higher multimode fiber counts in the same small area results in even less structural integrity for the molded piece.
Nevertheless, in an attempt to address the increasing industry need, companies have attempted to manufacture connectors for larger arrays using the techniques currently used to manufacture small array ferrules (i.e. ones with a single row of between two and 12 fiber holes) with little to no success. For example, one company is known to have made a 5×12 array ferrule and 5×16 array ferrule. One example of the 5×12 ferrule is shown in the photograph of FIG. 3 and both are described in Ohta et al., Two Dimensional Array Optical Fiber Connector, Fujikura Technical Review (2000). However, although not discussed in the article, applicants were informed that, in making those ferrules according to the prior art molding technique, they achieved such poor yields that the commercial cost of producing the pieces was deemed prohibitive—in that the problems encountered and extremely low yield would result in their being sold for some $500 each, if they could be sold at all. Moreover, the process was such that the molds for producing the pieces were destroyed in the process. As a result, they deemed arrays of that size (i.e. arrays of 5 rows) unmanufacturable using the molding processes then available. Other companies, when asked if they could provide similar large array ferrules, would not even attempt to do so, considering them unmanufacturable without even trying.
As described in the Ohta et al. paper, the ferrule also includes a row of guide grooves for each row of holes. In the ferrule of FIG. 3, the access way has been enlarged and the upper rows of guide grooves have been removed so that the holes for the fibers can be viewed through the access way of the ferrule.
FIGS. 4, 6 and 7 are further photographs of the 5×12 ferrule of FIG. 3 taken from different views.
FIG. 4 is a close-up photograph of the exposed row of guide grooves taken looking into the ferrule through an access way from the same angle as in FIG. 3. The purpose of the guide grooves is to facilitate fiber insertion and to support the fibers once inserted by effectively increasing the thickness of the face portion by up to an additional 1.5 mm or more.
FIG. 5 shows a simplified view of a portion 500 of a ferrule having a 3×4 array of fiber holes 502 and guide grooves 504, similar to those used in the ferrule of FIG. 3. The rows are stepped, with the lowest row 506 being the longest, and each successively higher row 508, 510 being slightly shorter. Depending upon the particular ferrule the guide grooves are semi-cylindrical or “V” shaped in cross section. During manufacture, fibers are inserted into the guide grooves of the lowest row, followed by the next higher row, etc. until all the desired fibers have been inserted. As their name implies, the guide grooves guide or direct the fiber into the fiber holes of the ferrule.
FIG. 6 is a closer photograph of the ferrule holes in the ferrule of FIG. 3 taken looking into the ferrule at an angle through the access way. As noted above, some of the rows of guide grooves have been removed so that several rows of holes are exposed for viewing. As can be seen in the photographs of FIG. 4 and, more clearly in the photograph of FIG. 6, there is visible variation in the size and shape of the holes as well as the walls separating one hole from another. These variations are due to the problems noted above. Depending upon the particular defect, the hole variation can inhibit fiber insertion, affect the pitch, or affect the inserted fiber angle (relative to other inserted fibers)—all undesirable results. In addition, although these holes are clearly visible in FIG. 6, in actuality, the fiber holes would be obscured from view by the guide grooves. In addition, the presence of the guide grooves makes it difficult, if not impossible, to fix a partially blocked or collapsed hole without damaging the ferrule.
FIG. 7 is a photograph of the same holes taken looking into the ferrule of FIG. 3 through the rear end of the ferrule. As can clearly be seen in this photograph, there is visible variation in the size and shape of the holes as well as the walls separating one hole from another including marked differences in hole size, partially blocked or collapsed holes and variation in wall thickness between adjacent holes.
As such, the prior art has been forced to do without commercial connectors for such large arrays, because such arrays can not be reliably created, and ferrules for use in commercial connectors for still larger format arrays are still considered unmanufacturable or prohibitively difficult for those in the art to even attempt. Moreover, since single mode fibers have an even smaller core diameter than multimode fibers and hence can have a smaller overall diameter, molding or machining ferrules for use in present form factor commercial connectors that will accommodate large arrays of single mode fibers is currently, for all practical purposes, considered equally prohibitive if not impossible—particularly on a cost effective commercially viable scale.
Thus, our attempts to find an entity that could mold a commercially available connector compatible plastic ferrule to accommodate an array of 5 rows×12 fibers/row or any large format array (in terms of number of rows over two, irrespective of fibers per row) left us discouraged and, like those in the art seeking similar pieces, to the conclusion that such ferrules could either not be made on a commercially viable scale or could not be made at all.
Thus, despite the strong and growing need for ferrules that can be used for large arrays of fibers, and the attempts in the art to fulfill those needs, the art has not been able to successfully do so. Moreover, to the limited extent anyone has even been able to mold the above 5×12 or 5×16 plastic ferrules at all, the ability to consistently and reliably produce such ferrules to address the need in the art at all, let alone in commodity item quantities, is elusive.